Method of making a composite metal article



Sept. 3, 1968 J. H. KEATING METHOD OF MAKING A COMPOSITE METAL ARTICLE 2 Sheets-Sheet 1 Filed Sept. 28, 1964 FIG. I

SPRAY COATING OF ALUMIN U M PRE HEAT BE LOW GRIT BLAST TO ROUGHEN SUR FACE VAPOR BLAST OR PICKEL TO CLEAN Fl G. 2

TO INFRA- RED FURNACE INVENTOR. JOHN H. KEATING ATTORNEYS P 1968 J. H. KEATING 3,400,010

METHOD OF MAKING A COMPOSITE METAL ARTICLE Filed Sept. 28, 1964 2 Sheets-Sheet 2 36 ATTORNEYS United States Patent 3,400,010 METHOD OF MAKING A COMPOSITE METAL ARTICLE John H. Keafing, Cleveland, Ohio, assignor, by mesne assignments, to Standard International Corporation,

Andover, Mass., a corporation of Ohio Filed Sept. 28, 1964, Ser. No. 399,451 3 Claims. (Cl. 117-933) ABSTRACT OF THE DISCLOSURE An improvement to the method of making a composite metal article comprising the steps of providing a substrate of a ferrous base metal, spraying a coating of aluminum to completely cover the substrate to a uniform thickness, subjecting the spray coated surface to a direct infra-red radiation of an intensity to raise the temperature at the ferro-aluminum interface to a temperature between 1200 F. and 1400 F., and holding the temperature for a period of time suificient to form an inter-metallic compound bonding phase having a thickness substantially less than the thickness of said sprayed coating of aluminum. The improvement comprises supplying the direct infrared radiation in relatively short periodic pulses.

The present invention relates to the art of metal casting and more particularly to the casting of one metal around a dissimilar metal imbedded therein.

The invention will be described with particular reference to the casting of ferrous metal inserts, such as cast iron or steel, either wholly or partly imbedded within an aluminum or aluminum alloy casting. However, it should 'be appreciated that the invention has much broader applications and may be used in a number of instances where metal castings have metallic inserts or where protective coatings are used.

Various methods have been proposed and used for bonding aluminum and aluminum alloys to ferrous base metals because of the advantages characteristics obtained in the resulting composite article over similar structures comprised entirely either of aluminum or of a ferrous metal. For example, aluminum may be applied to steel to protect the same against corrosion, steel inserts are often used as an internal reinforcement for aluminum castings where the properties of the latter material, such as its higher heat or electrical conductivity are important, or where the comparative strength to weight ratio of aluminum is desirable. Cast iron inserts may also be used in areas subjected to exceptionally heavy wear which could not be tolerated by aluminum.

In practically all of these cases it is highly important that a strong tightly adherent bond be formed between the ferrous and aluminum metals. This normally cannot be done by merely pouring aluminum against the exposed surface of the ferrous insert positioned in a mold. To avoid a chill, the insert must be heated to around 1000" F. whereupon its surface becomes oxidized which militates against the formation of a bond without using a flux. It has become common therefore to coat the insert while relatively unheated with a layer of aluminum prior to it being cast in a mold.

One process for doing this involves depositing aluminum or an aluminum alloy on a ferrous metal insert by dipping the unheated insert in a bath of molten aluminum so that a ferro-aluminum alloy reaction can take place between the two metals below the surface of the aluminum bath and protected from the atmosphere. Numerous problems are encountered with the prior art dipping process 3,400,010 Patented Sept. 3, 1968 which make it commercially unattractive as a standard method for coating inserts. For example the dipping bath must be at high temperature to quickly heat the surface of the cold inserts to a temperature favorable to the formation of an intermetallic phase between the aluminum and steel. The bonding phase is inherently brittle and must be kept substantially thinner than the resulting aluminum coating. Hence, a second aluminum dipping bath at a substantially lower temperature is usually required to arrest the formation of the intermetallic bonding phase.

Another problem associated with the prior art process is in bringing the molten aluminum into contact with the ferrous insert in such a manner that aluminum oxide dross on top of the bath is not picked up on the surfaces of the insert which are to be coated. This requires auxiliary equipment for separating and holding back the oxide scum and raises objectionable handling problems in transferring the insert from one bath to the next.

Still another disadvantage with the prior art process is the cost. Since iron goes into solution of aluminum rapidly as the temperature exceeds 1200 F., the high temperature aluminum dipping bath becomes contaminated with iron after a few inserts have been coated and must 'be discarded. Thus, replacing the bath with fresh aluminum substantially adds to the cost of the overall process.

Another problem is the percentage of coated inserts which have defective bonds in spite of the precautions taken of obtaining a uniform intermetallic bonding phase. In practice a high percentage of defective inserts result and since visual inspection cannot determine the presence of a defective bond, costly quality control procedures are a necessity.

The present invention contemplates a method which overcomes all of the above-referred to disadvantages and others and produces an intermetallic bonding phase which is strong and adherent virtually of the time.

In accordance with the present invention a method of making a composite metal article comprises providing an insert of a metal having a melting point substantially above 1600 F.; spraying a metallized coating on a surface of the insert metal which melts and alloys with the insert metal at a temperature below 1600 F.; subjecting the spray coated insert surface to radiant energy of an intensity and type sufiicient to raise the temperature at the interface of the two metals to the melting point of the coating metal and holding the temperature thereafter at least as high as the eutectic temperature of an alloy bonding phase of the two metals for a period of time sufficient to form a continuous alloy bonding phase having a thickness materially less than the thickness of the sprayed coating.

Radiant energy as used to describe the invention is distinguished from heat sources such as circulating hot air, or a molten bath. More particularly reference is made to infra-red radiation of sufficient intensity to heat the coated insert to an effective temperature at least several mils below the surface. In certain instances and particularly where the insert is a metal having a high electrical resistivity as compared to the coating, the radiant energy may be electromagnetic waves of a frequency and power sufficient to raise the skin temperature of the high resistivity insert to above the critical temperature.

More particularly, in accordance with the invention, where the insert is ferrous base metal and the coating is an aluminum or aluminum alloy, the coated surface of the insert is subjected to infra-red radiation of sufficient intensity to raise the temperature at the ferro-aluminum interface to above 1200F. when exposed to a steady pulse of such radiation lasting in the order of about one minute and such radiation pulse being repeated at a frequency that assures control of the formation of an intermetallic bonding phase at the interface of a thickness substantially less than that of the spray coating.

The principal object of the present invention is to provide an improved process for bonding one metal to another metal having a higher melting point.

A more specific object is to provide a process for bonding aluminum to a ferrous base metal by the creation of an intermetallic compound which acts as the bonding agent.

Another object is to provide a process for bonding aluminum to a ferrous base metal in which radiant energy is used to heat near the surface of the ferrous base metal to a critical temperature while the remainder of the base metal remains relatively cold.

Still another object is to provide a process for bonding aluminum to a ferrous base metal in which radiant energy is used to quickly heat the surface of the base metal to a critical temperature before the body of the base metal absorbs an appreciable quantum of heat.

Yet another object is the provision of a method of the character described subject to better process control and whereby a low percentage of rejects is attained.

Other objects and advantages will appear from the following description and drawings wherein:

FIGURE 1 is an illustration of a light weight metal article cast with a heavy metal insert imbedded therein;

FIGURE 2 is a diagram depicting the progressive steps of the process in accordance with the invention;

FIGURE 3 is a side elevational view of a rotary infrared furnace broken away to show the previously coated inserts being heated therein in accordance with the invention taken generally along line 3-3 of FIGURE 4; and

FIGURE 4 is a fragmentary plan view of the infra-red furnace shown in FIGURE 3.

Referring now to the drawings wherein the figures are for the purposes of illustrating a preferred embodiment of the invention only and not for the purpose of limiting same, FIGURE 1 shows an aluminum brake drum A of conventional design having a cast-in circular liner B of a metal such as cast iron which is bonded to the aluminum brake drum A. As is the practice, liner B is first coated on the outside with a thin aluminum layer and is then placed in a brake drum mold and molten aluminum is then poured in around the insert so as to form the brake drum A. Inasmuch as the present invention is not concerned with the technique of casting nor of the particular shape of the insert, no detail is deemed necessary in explanation of such casting process.

Referring to FIGURE 2, the cast iron liner B is subjected to the following process conducted in accordance with the invention. The process may be divided into three phases which include (1) preparation of the insert, (2) application of the coating to the insert, and (3) bonding the coating to the insert.

Preparation of the insert In preparing the insert B considerable flexibility is permissible since it is only necessary that the surface be sufficiently free of scale, rust, greaseand foreign material which would interfere with intimate physical contact between the metallized coating and the insert. While any one of the steps in preparing the insert may be omitted, depending upon the initial condition of the surface of the insert, it is preferred that the surface not be too smooth so that a good mechanical bond is achieved when the spray coating is applied. As depicted at 10, the first step is to grit blast the entire surface of the insert B on which the spray coating is to be applied. This removes scale and rust quickly and roughens the surface.

The next step, as depicted at 12, is to clean the insert of any foreign matter picked up in handling and to remove scale and loose surface oxides which remain. This step involves vapor blasting with a detergent and/ or pickling in a solution of any of the well known pickling baths.

Following this, the surface of the cleaned insert is preheated as depicted at 14 to a temperature of between 200 F. and 400 F. in a reducing flame. This preheating step is necessary only insofar as it insures a tight metallized coating application by warming the insert and driving off moisture. However, heating should not be so high as to cause a metallurgical change or in any way heat treat the insert.

Coating application Following the preheating step and while the insert is still warm, it is coated with a thin aluminum or aluminum alloy layer by conventional metal spraying techniques as indicated at 16. Metal spraying is a well known process of applying a coating of metal to another object. The metal, usually in the form of a wire, is melted by an oxyhydrogen or oxyacetylene blast or by an electric arc and is projected at a high speed by gas pressure against the object being coated. In this case, the metallizing wire is aluminum or aluminum alloy and not necessarily of the exact type used in casting the brake drum A, but preferably having properties similar to those of the casting alloy. A coating C is applied evenly over the surfaces which are to be bonded in the casting, in this case, the outer surfaces and edges of the liner B. The surface which is not to be bonded is stopped off or protected from the spray coating in any known manner. The sprayed aluminum metal strikes the warm surface of the insert and covers it rather uniformly with a thin layer approximately 0.005 inch thick, but generally not to exceed 0.1 inch. The minimum coating thickness actually is that necessary to attain complete coverage of the suface involved, In accordance with the invention, the insert B having the aluminized coating C thereon is then transported to an infra-red furnace 20, such as that shown schematically in FIGURES 3 and 4.

Bonding the coating to the insert Bonding of the coating C to the insert B is carried out preferably in the rotary infra-red furnace which has inner and outer refractory walls 22, 23 defining an annular chamber 24. Appropriate entrance and exit doors, not shown, permit conveyorized movement of the inserts through the furnace. A circular conveyor 26 carries fixtures 28 spaced at intervals thereon through the heating chamber 24 in the direction indicated by the arrow in FIGURE 4. Each aluminized insert B is suitably supported on a shaft 30 of each fixture 28 which shaft is adapted to rotate relative to a base 32 resting on the conveyor 26 so as to expose the insert B to uniform heating. Any suitable means may be used to drive the conveyor 26 and rotate shafts 30 as indicated generally at numeral 32. It should be appreciated that the details of the furnace 20 or the fixtures 28 are not a part of this invention and almost any refractory heating structure and fixturing may be used consistent with the invention.

Arranged in the walls 22, 23 of the furnace 20 are a plurality of infra-red heaters 35 which are staggered vertically and spaced circumferentially to form opposing heater banks 36. The heaters 35 are of a standard gas fired infra-red type commonly employing a ceramic body having a cup shaped end with an Inconel wire mantle 37 in the center for causing ignition of the gas fed through lines 38-through walls 22, 23 of the furnace. Heaters of this type are commercially available and are rated in heat density, at least within a short distance from the mantle, as high as 1600 F. Each heater 35in the banks 36 is inclined with respect to a radial plane through the center of the furnace such that the heaters in the outer wall 23 are aimed toward the oncoming inserts B while those on the inner wall 22 are inclined slightly in the opposite direction to heat the insert as it moves away.

A number of opposed burner banks 36 spaced circumferentially in the walls 22, 23 around the furnace 20 may be employed. It is significant to note that in a circumferential direction between banks 36 there is relatively little heating since the intensity of the infra-red waves drops off rapidly as a function of the square of the distance from the source. Thus, in the intervening circumferential spaces 41 between banks 36 there is little or no direct heating of the inserts B with the result that they give off heat rather than absorb it from the ambient atmosphere of the furnace.

Thus, as each aluminized insert B is moved on the conveyor 26, the coating C will receive periodic pulses of infra-red radiation as it passes through the successive heating zones 42 of the furnace. The circumferential spacing of the banks 36 and the speed of the conveyor 26 may be arranged such that the duration of any heating pulse is regulated. It has been found that a pulse duration in the order of about one minute in a heating zone 42 followed by a similar period in a cooling zone 41 sequentially repeated provides control of the formation of an intermetallic bonding phase wherein the total heating cycle takes from 3 to 5 minutes and for smaller parts, much less time. It is preferable that the surfaces being heated be of a dull or dark coloration so as to absorb maximum calories per square inch per unit of time of exposure to the infra-red waves. Thus, while aluminum is a bright metal, the oxide coating which forms rapidly on its surface is dull gray and serves as a relatively black body with respect to infra-red waves.

The intensity of the infra-red waves at the first pulse of energy is sufiicient to raise the temperature of the coating C to approximately 1200 F. The rate of thermal conductivity of aluminum is approximately three times that of cast iron, thus as the infra-red waves are absorbed in the surface of the coating C, the heat is conducted rapidly away from the surface until the entire coating thickness and skin of the insert B are quickly raised to the melting point of aluminum but well below that of iron. This occurs during the initial pulse of energy received in a heating Zone 42 from the infra-red banks 36, such that the first vestiges of molten aluminum appear in the coating C at this time. The melting Within coating C is protected from oxidation by a heavy layer of aluminum oxide on the surface which is too refractory to melt. At the critical temperature when iron goes into solution with aluminum, i.e., the eutectic of about 1200 F., the formation of an initial iron-aluminum intermetallic compound phase occurs at the interface.

Thereafter the insert B is moved through successive heating and cooling zones 41, 42 whereupon the thin intermetallic bonding phase is built up at the interface between the coating C and ferrous insert B.

Almost immediately as each insert moves beyond the range of the heaters 35, it begins to give off heat to the atmosphere of the furnace. The heat absorbed by the skin of the insert B, is drawn back up into the coating C so that the insert remains at least above the eutectic temperature near the interface.

Presumably, the temperature after the first pulse of radiation is somewhere in the order of 1200 F. and very little bonding phase will be formed. At the second pulse of energy, heat is again poured rapidly into the surface of coating C by the absorption of the infra-red waves, thus reversing the flow of heat back into the skin of insert B. After the second energy pulse, the skin temperature of the insert is restored and probably increased in temperature to about 1400 F. whereupon rapid development of the bonding phase takes place. The alternate heating and cooling continues with the surface of the insert B acting as a heat-sink and continuing to promote the development of the intermetallic compound layer until a thin bonding phase is formed which extends substantially continuously between the coating C and insert B.

It is important to understand that only the metallized coating C of aluminum and the upper skin of the insert are substantially heated by the infra-red waves and that the substrate of the insert B remains relatively cool throughout the process. This insures close control of the formation of the bonding phase, the thickness of which should be substantially less than the thickness of coating C to avoid brittleness.

It has been determined that the bond formed by practicing the subject invention is strong and uninhibited by the presence of any aluminum oxide film at the interface as shown by photomicrographs and by destructive testing. For example, aluminum parts having steel inserts were solution heat treated at 1000 F. for 8-10 hours followed by a rapid quench. Such a severe thermal shock would cause cracking and spalling of the aluminized coating, especially due to the great difference in coeflicients of thermal expansion between iron and aluminum, unless a strong, continuous bonding phase was present between the two metals. Attempts thereafter to separate the insert from the aluminum casting invariably resulted in a failure within the body of aluminum and not at the ferro-aluminum interface.

While infra-red radiation has been described as a preferred method of heating, it is clear that induction heating may be employed since it is essentially a surface heating technique. Also, while the description has been with reference to coating iron with aluminum, the proc ess is equally applicable to coatings of zinc on iron or aluminum on copper base metals for example. Modifications of the process may be made by persons skilled in the art without departing from the invention as defined in the appended claims.

I claim:

1. In the method of making a composite metal article comprising the steps of providing a substrate of a ferrous base metal, spraying a coating of aluminum to completely cover the substrate to a uniform thickness of about 0.005 inch, subjecting the spray coated surface of the article to direct infra-red radiation of an intensity sufficient to raise the temperature at the ferro-aluminum interface to a temperature between 1200 F. and 1400 F. and, holding such temperature for a period of time sufficient to form an intermetallic compound bonding phase at the interface having a thickness substantially less than the thickness of said sprayed coating of aluminum; the improvement comprising:

supplying the said directed infra-red radiation in periodic pulses which have a duration of less than one minute.

2. In the method of making a composite metal article comprising the steps of providing a substrate of a ferrous base metal, grit blasting the surface of the substrate which is to be coated, heating the substrate to a temperature in the range of 200 F. to 400 F., spraying a coating of aluminum on the substrate to a thickness of approximately 0.005 inch, subjecting the spray coated surface to direct infra-red radiation of an intensity sufficient to raise the temperature at the ferro-aluminum interface to between 1200 F. and 1400 F., and holding such temperature for a period of time in the order of five minutes so as to form an intermetallic bonding phase at the interface having a thickness substantially less than the thickness of said sprayed coating; the improvement comprising:

supplying the said direct infra-red radiation in pulses followed by cooling periods each of a duration in the order of one minute.

3. In the method of making a composite metal article comprising the steps of providing a substrate of a ferrous base metal, spraying a coating of aluminum to completely cover the substrate to a uniform thickness in the range of about 0.005 inch to about 0.1 inch, subjecting the spray 8 References Cited UNITED STATES PATENTS 2,845,366 7/1958 Schroeder 117-105 X 2,908,073 10/1959 Dulin 29-1962 3,033,705 5/1962 Hanink et a1. 117105 X 3,305,384 2/1967 Kenderi 117-105 X ALFRED L. LEAVITI, Primary Examiner.

10 J. H. NEWSOME, Assistant Examiner. 

